During sleep, human beings typically enter rapid eye movement (REM) sleep (e.g., dream sleep) during approximately 25% of their overall sleep time. During an average life-expectancy, one could expect to sleep for approximately 20 years and dream for approximately 5 years. While the vast majority of people experience dreams passively as a quasi-random set of experiences that happen to them and over which they have very little, if any, control, some people experience lucidity during their dream state. A lucid dream is a state in which one becomes aware that one is dreaming, and in many cases, they can control the dream experience. The ability to regularly have lucid dreams is quite rare. Some people, through the use of various mental exercises, are capable of inducing a lucid dreaming experience, though this often requires extensive practice and concentration, and even still only results in occasional lucid dreams.
During typical REM sleep, one experiences a primary state of consciousness that is concerned only with the immediate present. During wakefulness, human beings experience a secondary state of consciousness that introduces higher order cognitive functions such as self-reflective awareness, abstract thinking, and access to thoughts of both past and future. One way to think of lucid dreaming is as a state of sleep in which both primary and secondary states of consciousness exist simultaneously which allows the subject to become aware of the fact that he or she is dreaming during the dream. This awareness or lucidity can also provide the subject with the ability to exert control over the ongoing dream plot.